.alpha.-Olefins are useful as a comonomer for modification in the field of preparing polyolefins or as a plasticizer or a surface active agent after they are alcoholized.
Generally, such linear .alpha.-olefins are prepared by oligomerizing ethylene in the presence of a catalyst. As for the catalyst there is known, for example, a binary catalyst which consists of titanium tetrachloride and ethylaluminium dichloride. Also, a process for increasing the yield and selectivity of .alpha.-olefins using a catalyst which consists of a third component in addition to the above-described binary catalyst composition is known.
On the other hand, binary catalysts with increased activities have recently been proposed which comprises zirconium (Zr) instead of the above-described titanium compound. For example, Japanese Patent Application Laid-Open Nos. 109428/83, 201729/83 and 113138/83 and Japanese Patent Publication No. 30042/75 disclose binary catalysts whcih consist of a Zr compound and an Al compound.
However, processes for the oligomerization of ethylene using the above-described binary catalysts comprising Zr have caused various problems. For example, final products contain wax fraction in large amounts or the yield of oligomers having a small number of carbon atoms, e.g., those having about 4 carbon atoms, is increased and the purity of the .alpha.-olefins obtained is very low. Further, long continuous operation of production unit is difficult since wax fraction is formed in a large amount or for other reasons.
U.S. Pat. No. 4,486,615 discloses a catalyst composition consisting of a binary catalyst composed of a Zr compound and an Al compound and a Lewis base such as a tertiary amine, a secondary amine, an ether, a phosphine oxide, an alkyl phosphate, an aryl phosphate, a sulfoxide, etc. which is added as a third component in order to increase the activity of the binary catalyst.
However, although it is described in U.S. Pat. No. 4,486,615 that a linear .alpha.-olefin having high purity as high as 99.5% can be prepared using the catalyst composition containing triethylamine as the most preferred example of the Lewis base it has been confirmed by the present inventors that use of the catalyst compositions containing Lewis bases referred to in U.S. Pat. No. 4,486,615 other than triethylamine results in the preparation of linear .alpha.-olefins having low purity and that a large amount of the above-described ternary catalyst containing triethylamine is used in U.S. Pat. No. 4,486,615.